Polymeric films and articles are extensively used in optical applications. One major problem with the use of such materials is reflective losses at the substrate/air interface, resulting in lower intensity of transmitted light. Issues of reflective losses across multiple interfaces can be addressed by adjusting the refractive indices of the films. One such example is cured film of urethane acrylate resin, which is widely used as protective coat in variety of applications involving display devices. Although, urethane protective coats have excellent transparency, hardness and scratch resistance, it is difficult to modify their refractive indices due to limited choices of building materials that are available for optical applications. An alternative means of modifying refractive index is to use small amounts of miscible additives, which do not alter other fundamental properties such as transparency, hardness and scratch resistance.
High refractive index values of metal compounds make them ideal candidates as additives to boost refractive indices of organic polymeric materials. For instance, Arpac et al in U.S. Pat. No. 6,291,070 describe use of several nanoscale inorganic particles to create molded articles of varying refractive indices. Practical utility of inorganic particles in boosting refractive index is greatly restricted by the limited compatibility between such particles and organic polymeric matrices. Processes such as “micronization” can produce nanoparticles with relatively high dispersion to some extent but there is a practical limit to the size achievable economically by “micronization”. For applications where transparency is important, the particle size must be smaller than the wavelength of the light in order for the material to be transparent. Sol-gel or solution-colloidal phase reactions are alternative means of generating very fine particles of metal oxides, but the nature of the small particles often leads to their agglomeration, causing increased hazing and scattering of a transparent article over time.
Issues of agglomeration of fine particles can be addressed through chemical surface reactions. For instance, inorganic particles, described by Arpac et al. in U.S. Pat. No. 6,291,070, were surface-treated with hydrolysable silane containing at least one polymerizable and/or polycondensable group. Chisholm et al. in U.S. Pat. No. 6,844,950 also describe the use of nanoparticles of ethylenically unsaturated compounds of zirconium and titanium. Similarly, Arney et al. in U.S. Pat. No. 6,432,526 describes the use of metal oxides modified with dispersing aids for improved compatibilization with organic materials. The main difficulty with this approach is that the actual nanoparticle compositions are changed by attaching these modifying species to them. Moreover, the metal concentration in any subsequent formulations is decreased by the presence of these organic functional groups. Most critically, the issues of hazing and light scattering after the article has been exposed to prolonged storage are not completely solved due to the limited shelf life of surface modified metal particles. Designing metal-containing compositions with homogeneous dispersion in the final article or the polymerizable fluid and long shelf life stability, therefore, continues to be a challenge.
Use of discrete metal compounds as processing aids and curing agents in the processing of certain types of elastomers and some dental compositions is known. For instance, Nagel et al in U.S. Pat. No. 6,194,504 describe the use of metal salts of acrylic acid as processing aids to improve dispersion of such curing additives in butadiene, natural rubber and EPDM based elastomers. Fabian in U.S. Pat. No. 6,553,169 and Shustack et al. in U.S. Pat. No. 6,656,990 describe the use of less than 0.5 weight-percent of titanates and zirconates as energy curable coupling agents to improve adhesive properties and dispersion of pigments. Similarly, use of zirconium-based acrylate as coupling agent between amorphous calcium phosphate and polymeric matrices has been reported by Skrtic et al [Biomaterials 24 (2003) 2443-2449]. None of the art reported above teaches how to create an optically clear film or article with excellent physical and mechanical properties, especially ones with high refractive index, and improved shelf life from compositions containing discrete metal-containing functional precursor units.
Conventional methods of patterning into metal-based substrates involve complex multi-step processes including deposition of the nascent metal-compounds on suitable substrates and patterning into such compounds.
Techniques commonly employed to deposit metals or metal oxides are spin-on deposition of metal precursor (sol-gel), evaporation, sputtering, and chemical vapor deposition (CVD). Each of these techniques has several limitations and hence has so limited commercial viability.
Sol-gel is one of the most common methods of depositing a metal oxide layer. The method employs spin-on coating of a sol precursor dissolved in a suitable solvent followed by heating the substrate to a high temperature to convert the precursor film into metal oxide. The method is not very practical in the sense that it employs quite high temperatures. High stresses related to cycles of heating and cooling involved in the process lead to defectivity. Moreover, additional processing steps are required to pattern small features into the material.
Deposition by evaporation involves heating of metal-compound to be deposited to high temperatures. Vapors of such materials are condensed on the substrate under vacuum using a screen or shadow to form fine patterns of the material. Deposition by evaporation has limited commercial potential due to high temperatures and high vacuum requirements.
Deposition by sputtering involves vaporization of the material to be deposited by bombarding with high-energy atomic radiation. Similar to Evaporation, the vapors of the materials can be deposited on the substrate by condensation. Utility of the process is limited due to high energy requirements and lack of precision in controlling film properties.
The process of deposition by CVD is even more expensive than sputtering or evaporation due to additional costs associated with the specialized equipment required for the chemical reactions prior to material deposition.
Formation of fine patterns into such metal-containing layers is achieved by additional multiple steps of imaging and etching of a photosensitive film deposited on such materials. First a photoresist or photosensitive film is applied on the metal-containing layer and dried at an appropriate temperature to remove a majority of casting solvent followed by image-wise exposure to actinic radiation to which such photoresist material is sensitive In case of a positive acting photoresist, the exposed area of the film undergoes chemical reaction rendering it soluble in an alkaline developer. The action of developer leaves behind a fine pattern of the photoresist material. In the case of a negative acting photoresist, the exposed portion of the film undergo chemical reaction rendering it insoluble in a solvent suitable for removing unexposed area of the film, leaving behind a fine pattern of the photoresist film, which acts as an etch mask to transfer pattern into underlying metal-containing material.
Another method of forming fine patterns involves depositing a non-photosensitive metal-containing film into a patterned substrate. The non-photosensitive metal-containing film is etched back to resolve the underlying pattern.
The techniques of metal-compound deposition as well as patterning are cumbersome and expensive. Ultra high purity materials are required for successful deposition. Moreover, the resolution of the pattern formed by some of the techniques is quite limited. Therefore, methods involving direct patterning into metal-containing layers are desired.
A photoresist-free, negative-tone method of direct patterning into metal-containing materials is described in U.S. Pat. Nos. 5,534,312; 6,071,676 and 6,972,256. The metal complex used in the method is photosensitive and undergoes low-temperature reaction in the presence of light of particular wavelength rendering the exposed portion insoluble in solvent/developer. Disadvantages of this method include limited choices of the starting metal-containing materials which demonstrate a sharp switch in solubility behavior upon exposure requiring very high exposure doses and often treatment of harsh solvents to remove unconverted material. Such harsh solvents also attack the exposed area, destroying pattern fidelity. This technique involves metal compounds that show significant absorbance at the exposure wavelength. High absorbance at the film surface leads to less light penetration into the bulk of the film resulting in non-uniform photochemical reaction; hence, chemical composition of the film is quite heterogeneous across the film.
More recently, Hill et al in U.S. Pat. No. 7,176,114, describe positive-tone pattern formation into metal-containing precursor layers. Materials used in the positive-tone method show sharper solubility contrast between the exposed and un-exposed areas of the film. However, most of the disadvantages noted with the negative tone-methods such as requirement of high energy dose and high exposure times make this method less pragmatic for real production environment.